US 20040257840 A1
A power conversion system which uses a plurality of transistors to convert DC power to AC power is made more efficient by the use of liquid refrigerant to cool the transistors. A plurality of passages are formed in a cold plate on which the transistors are mounted, and liquid refrigerant is flashed into the passages to cool the cold plate and the transistors. The liquid refrigerant can by derived from a condenser from an organic rankine cycle or a vapor compression cycle.
1. A power conversion system for converting electrical power from a high frequency level to a lower frequency level comprising:
means for rectifying the power of a high frequency AC source to create a direct current;
means for converting said DC current to AC current of a lower frequency than that of said AC source, said converting means including a plurality of switching mechanisms mounted in heat exchange relationship with a cold plate; and
liquid refrigerant circulation means for circulating liquid refrigerant through said cold plate to provide cooling thereof.
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10. An electrical power generating system comprising:
a turbine for receiving a gaseous medium and converting its energy to motive power;
an electrical generator drivingly connected to said turbine for producing AC power at a relatively high frequency;
a power converter connected to said generator for converting said high frequency AC power to lower frequency AC power with the use of a plurality of switches; and
means for cooling said plurality of switches by circulating liquid refrigerant through a structure on which said switches are mounted.
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20. A method of improving the performance of a power converter of the type having an invertor for converting direct current to an alternating current by way of a plurality of switches, comprising the step of:
providing a cold plate;
mounting a plurality of transistors on said cold plate in heat exchange relationship therewith; and
introducing a flow of liquid refrigerant in the vicinity of said cold plate and allowing it to flash to a vapor while cooling said cold plate and said plurality of switches.
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 This invention relates generally to power conversion systems and, more particularly, to a method and apparatus for cooling power converters.
 In the power generation industry, high speed generators are used to generate electrical power at relatively high frequencies (typically a number of times higher than the grid frequency), and power converters are used to convert the high frequency AC power down to an AC grid frequency of 50 or 60 Hz. This is accomplished by rectifying the high frequency AC signal and then generating a new sinusoidal AC wave of the desired frequency. The creation of the new wave involves the use of integrated gate bipolar transistors (IGBT's), which are switched at a carrier frequency rate, which is an order of magnitude higher than the power converter output frequency. Because of the necessary repeated on and off switching of the IGBT's, they generate a substantial amount of heat which must be dissipated. Without cooling this dissipated heat, the temperatures of solid state junctions in the IGBT's would exceed the maximum values, and the transformers would be permanently damaged. Accordingly, all high frequency power conversion equipment typically applies some type of cooling in order to control these junction temperatures.
 The cooling requirement increases with power capacity in the IGBT's, and, at a given switching frequency, as the amount of current going through an IGBT is decreased, the heat that is developed will be decreased. Accordingly if the temperature of the solid state junction can be reduced, then the current being switched on and off through the junction can be increased to thereby enhance the power density of the power converter. Further, acceptable IGBT lifetime can be assured if the temperature of the solid state junction is kept below a certain maximum value.
 Progress has been made in IGBT design to enable forced-convection air-cooled finned cold plates in removing the heat from the transistors. However, forced-convection air-cooling of finned cold plates is relatively ineffective due to the poor heat transfer between the cooling air and the finned cold plate.
 It is therefore an object of the present invention to provide an improved cooling method and apparatus for a power converter.
 Another object of the present invention is the provision in a power converter for more effectively cooling the transistors therein.
 Another object of the present invention is the provision in a power converter for reducing the temperature of the solid state junctions while increasing the current flow therein.
 Yet another object of the present invention is to improve the life of transistors in power conversion systems.
 Still another object of the present invention is the provision for a power conversion system that is economical to manufacture and effective and efficient in use.
 These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.
 Briefly, in accordance with one aspect of the invention, liquid refrigerant is used to reduce the temperature of the transistors and increase the current flow therethrough.
 In accordance with another aspect of the invention, liquid refrigerant is caused to flow through internal channels of a cold plate on which the insulated gate bipolar transistors are mounted. In this way, the heat from the transistors is more effectively dissipated such that the transistors can carry greater currents and operate more efficiently.
 By yet another aspect of the invention, the liquid refrigerant is taken from the discharge of a pump in an organic rankine cycle system and is routed to a power converter receiving the output of a high frequency generator driven by the organic rankine cycle turbine.
 By yet another aspect of the invention, the liquid refrigerant is taken from the discharge of a pump in an organic rankine cycle system and routed to a power converter receiving its electrical power from a generator driven by a microturbine.
 By still another aspect of the invention, the liquid refrigerant is taken from the condenser of a conventional vapor compression cycle, and is allowed to expand into the cold plate, after which the resulting vapor is caused to flow back to the compressor of the vapor compression cycle.
 In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.
FIG. 1 is an organic rankine cycle power generation system in accordance with the present invention.
FIG. 2 is a schematic illustration of the function of a power converter in accordance with the prior art.
FIG. 3 is a schematic illustration of the cooling of a power converter in accordance with the prior art.
FIG. 4 is a schematic illustration of the cooling arrangement for a power converter in accordance with the present invention.
FIG. 5 is a schematic illustration of the cooling of a power converter in a microturbine system in accordance with the present invention.
FIG. 6 is a schematic illustration of the cooling of a power converter with a conventional vapor compression cycle in accordance with the present invention.
 Referring now to FIG. 1, a rankine cycle system is shown to include an evaporator/cooler 11, a turbine 12, a condenser 13 and a pump 14 arranged for the serial flow of refrigerant therethrough as shown by the arrows.
 The evaporator/cooler 11 is commonly a boiler having a significant heat input, and the energy source that is applied as waste heat can be of any form of waste heat that may normally be lost to the atmosphere. For example, it may be a small gas turbine engine (<500 kw), known as a microturbine or a larger industrial gas turbine engine (>500 kw). In either case, the heat is derived from the exhaust gas thereof. Other possible sources of waste heat are from the exhaust gases and/or cooling liquids from internal combustion engines. Finally, the energy may be derived from geothermal sources or from landfill flare exhausts wherein the hot fluids are applied directly to the boiler to produce refrigerant vapor or applied indirectly by first using those resource fluids to drive an engine which, in turn, gives off heat which can be used as described hereinabove.
 In operation, the evaporator/cooler 11 vaporizes the refrigerant which then passes to the turbine 12 for providing motive power thereto. Upon leaving the turbine 12, the low pressure vapor passes through the condenser 13 where it is condensed by way of heat exchange relationship with a cooling medium. The cooled condensed liquid is then pressurized and circulated to the evaporator/boiler by a pump 14 to complete the cycle.
 Drivingly connected to the turbine 12 is a high frequency generator 16. In order to be efficient, the turbine 12 must operate at high speeds, and preferable the generator 16 is connected in a direct drive relationship such that the generator 16 must also operate at relatively high speeds. As a result, and as desired for greatest efficiency, the power generated by the high frequency generator 16 is typically a number of times greater than the frequency of the grid (i.e. 50 or 60 Hz) that is to receive the generated power. It is therefore necessary to provide a power converter 17 to convert the high frequency power to a lower frequency power that can be applied directly to the grid.
 Referring to FIG. 2, there is shown a functional diagram of what occurs within the power converter 17. Briefly, the high frequency AC signal is passed through one or more rectifiers 18 that convert the power to DC, and then the DC signal is fed to an invertor 19 which generates the new sinusoidal AC wave of the desired low frequency.
 The invertor function is accomplished by the sequential operation of a great number of switching mechanisms commonly referred to as IGBT's (insulated heat bipolar transistors). Because of the required rapid switching operation, the IGBT's tend to generate a great deal of heat which, if not dissipated, will quickly burn out the transistors. It has therefore become conventional practice to mount the IGBT's 21 on a cold plate 22 having a plurality of cooling fins 23 as shown in FIG. 3. In order to dissipate the heat from the cold plate 22 a cooling fan 24 is directed to circulate cool air over the fin 23. Although effective, this approach has been recognized by the applicants as inadequate in providing the degree of cooling that is desirable in order to allow for the desired power densities in the power converter 17.
 Referring again to FIG. 1, it will be seen that at the output of the pump 14 a relatively high pressure cold refrigerant is readily available and can be effectively used in the cooling of the power converter components. A line 26 is therefore provided to conduct a relatively small amount of cold, high pressure, liquid refrigerant to the power converter 17 where it is expanded into a plurality of internal openings 27 of the cold plate 22 as shown in FIG. 4. The flashing of the refrigerant into the cold plate 22 quickly and substantially reduces the temperature of the cold plate, which is in contact with the IGBT's 21, and reduces the IGBT solid state junction temperature dramatically, e.g. by 50 to 100° F. depending on the design. This, in turn, allows more current to be passed through the IGBT'S, such that the power switching capacity of the solid state hardware increases substantially thereby increasing the power density by 50 to 100%, thus reducing the cost of power conversion equipment by the same percentage.
 After the refrigerant passes through the cold plate 27 the resulting refrigerant vapor is passed along through the condenser 13 by way of line 28 as shown in FIG. 1.
 Referring now to FIG. 5, another embodiment of the present invention is shown to include not only the organic rankine cycle system as described hereinabove, but also a microturbine that can be used in combination with the organic rankine cycle system. For example, the microturbine may burn natural gas or landfill methane gas in its combustion chamber, and with use of the turbine, convert that energy into electrical energy which may be transferred to the grid. In turn, the exhaust from the microturbine may be used to heat the boiler 11 of the organic rankine cycle system.
 The microturbine system as shown in the left side of FIG. 5 includes a burner or combuster 29, a turbine 31, and a generator 32 which operate in much the same manner as the turbine and the generator in the organic rankine cycle system as described hereinabove. Further, the power generated by the generator 32 is typically of the high frequency type as described hereinabove such that it is necessary to pass it through a power converter 33 to convert the power to a lower frequency form that can be assimilated into the grid.
 The gas turbine requires compression of ambient air and fuel (e.g. natural gas or methane) prior to ignition in the burner 29. This is accomplished by a compressor 34 which is driven by the shaft that is common to the generator 32 and the turbine 31.
 The power converter 33 is of the same type as the power converter 17, and the cooling arrangement as shown in FIG. 4 is substantially the same, with liquid refrigerant passing along line 36 to the cold plate of the power converter 33, and the refrigerant vapor then passing back to the condenser 15 by way of line 37.
 The cooling requirement of a power converter is around 3% of the power being converted. A typical air cooled air conditioning device having a coefficient of performance of around 3, will therefore need only 1% of the power generated to perform the cooling thus doubling the power density or capacity of th power converter.
 The vapor compression system as shown in FIG. 6 is a relatively small system on the order of a room air conditioner for a 200 kw microturbine, and can be easily and economically adapted for use to cool a power converter and in doing so will substantially increase its power capacity so as to being about substantial economic gains.
 While the present invention has been particularly shown and described with reference to preferred and alternate embodiments as illustrated in the drawings, it will be understood by one skilled in the art that various changes and detail may be effected therein without departing from the true sprit and scope of the invention as defined by the claims.